How do I tone down the baby envy?

A large percentage of the women I know are currently pregnant or have given birth in the last year. I’m sure most of this is because of my age, this is when most people have kids… so it is only natural that I would know a lot of people going through this right now.
I’m genuinely happy for them, but at the same time I am resentful and jealous… and finding it harder and harder to share in the elation. I don’t want to feel that way, because it isn’t right, but I can’t seem to help it anymore. Help? :\